The regulators to be tested commonly involve a voltage sensing circuit for sensing a voltage applied to the regulator and for switching a field switching device to a conductive or nonconductive condition to control field current to a machine to control the output voltage thereof. When the field switching device is controlling an alternator or the like and is in a conductive condition, field current flows to the field and when in a nonconductive condition, the field current is off. The voltage sensing circuitry normally includes semiconductor components, e.g., a Zener diode and a control transistor for switching on and off the field switching device which is commonly a transistor or transistors, e.g., a Darlington circuit arrangement. Also, alternator regulators are commonly provided with a diode circuit which is connected directly to the AC output of the armature windings of the alternator. The purpose of the diode circuit is to provide a voltage when the alternator is rotating, particularly on starting, to operate a switching circuit, commonly a transistor, in the regulator for causing the application of the voltage across the terminals of the regulator to the voltage sensing circuitry and to the field switching device. The regulator then operates to switch on the field switching device until the voltage rises to a maximum voltage regulation level. At this time, the voltage sensing circuit switches the field switching device off until the voltage drops to a lower voltage regulation level at which the voltage sensing circuitry again switches on the field current. The voltage sensing circuit commonly includes an adjustable voltage dividing circuit, e.g., a potentiometer, for adjusting the voltage at which the switching occurs as well as other components such as a capacitor for effecting fast switching and proper operation of the circuitry and a zener diode which switches on and off at the high and low voltage regulation levels.
Various types of testers have been utilized to test voltage regulators. One type of known tester is a static tester which utilizes a variable power supply and voltmeter, with the variable power supply being adjustable to various voltages to determine the switching voltages of the regulator. However, the measurement of the voltage setting in this type of tester is generally unpredictable with errors of up to 0.5 volts.
Dynamic testers have also been utilized. These testers have been open loop testers which periodically apply a voltage such as an A.C. voltage to the regulator to force the regulator to switch at the applied frequency. The measurement of voltage settings of such a tester will not commonly provide a performance test for various vital parts of a regulator, for example, capacitors.